Method and apparatus for determining the permeability characteristics of a porous or fissured medium

ABSTRACT

A borehole is formed in the medium and divided along its axis into three adjacent cavities, separated from one another and comprising two protecting end cavities enclosing an intermediate measuring cavity. A flow of a liquid is produced in each of the cavities and in the regions of the corresponding medium. Measurements are effected of the flow-rate of liquid flowing in the intermediate cavity and of the liquid pressure in intermediate cavity and in the corresponding region of the medium, at known distances from the axis of the borehole.

United States Patent [191 Louis 1 METHOD AND APPARATUS FOR DETERMININGTHE PERMEABILITY CHARACTERISTICS OF A POROUS OR FISSURED MEDIUMFLOWMETER PIEZOMETRIC CELL Mar. 18, 1975 2,379,138 6/1945 Fitting, Jr.et a1. 73/155 2,414,913 l/l947 Williams 73/151 X 2,605,637 8/1952Rhoades 73/151 2,781,663 2/1957 Maly et a1... 73/151 3,163,211 12/1964Henley 73/155 X 3,224,267 12/1965 Harlan et a1. 73/155 PrimaryExaminer-Jerry W. Myracle Attorney, Agent, or Firm-Larson, Taylor &Hinds [57] ABSTRACT A borehole is formed in the medium and divided alongits axis into three adjacent cavities, separated from one another andcomprising two protecting end cavities enclosing an intermediatemeasuring cavity. A flow of a liquid is produced in each of the cavitiesand in the regions of the corresponding medium. Measurements areeffected of the flow-rate of liquid flowing in the intermediate cavityand of the liquid pressure in intermediate cavity and in thecorresponding region of the medium, at known distances from the axis ofthe borehole.

7 Claims, 5 Drawing Figures METHOD AND APPARATUS FOR DETERMINING THEPERMEABILITY CHARACTERISTICS OF A POROUS OR FISSURED MEDIUM Theinvention relates to a method for determining the permeabilitycharacteristics of a porous or fissured medium, especially of soils andof rocks, according to which there is used a flow of liquid, in themedium, caused by the injection or pumping of the liquid into aborehole.

The invention relates more particularly, because it is in this case thatits application seems to present the most advantage, but notexclusively, to a method for the determination of the hydraulicparameters of the sub-soil.

It has already been proposed to carry out hydraulic tests in situ, on alarge scale, of which the results are more significant than those oflaboratory trials, but the interpretation of the'results of these trialsis rendered very difficult by the fact that, on the one hand, the testcavity is oftenbadly defined and that, on the other hand, the nature ofthe flows is not known. Under these conditions, a strict interpretationof the results cannot be made since the contribution of each directionalpermeability is poorly known or, for anisotropic media, the directionalor principal permeabilities are unequal. Even for isotropic media, thecorrect interpretation of the tests rem ains'delicate since the relativeimportance of planar or cylindrical radial flows and of spherical flowsis not known, the equations being different for each type of flow.

It is a particular object ofthe invention, to render the abovesaidmethod such that it responds to the various exigencies of practicebetter than hitherto and especially such that it no longer has, or hasto a lesser degree, the above-mentioned drawbacks of the prior art.

According to the invention, a method for determining the permeabilitycharacteristics of a medium, especially of soils and of rocks, accordingto which there is used a flow of liquid, in the medium, caused by theinjection or pumping of liquid into a borehole, is characterised by thefact that, on the one hand, at least a portion of the borehole is alongthe axis of this hole, into three adjacent cavities, separated from oneanother, comprising two protective end cavities, surrounding anintermediate measuring cavity, that on the other hand, there is producedin each of the cavities, and in the corresponding regions of the medium,a flow of liquid and that on the other hand lastly, there is effected ameasurement of the flow-rate of liquid flowing in the intermediatecavity and measurements of the pressure of the liquid in this cavity andin the corresponding region of the medium. at known distances from theaxis of the borehole.

Preferably, the axis of the borehole is parallel to an assumed principaldirection of permeability, in the case of media considered ascontinuous.

In the case of a fissured discontinuous medium, having three families ofparallel fissures, the axis of the borehole is taken as parallel to thedirection of the intersection of two families of fissures.

The invention also relates to an apparatus for the application of thepreviously defined method.

In a first embodiment, such an apparatus is characterised by the factthat it comprises at least two separate tubular pipes, adapted to beintroduced into the borehole and at least two closures adapted to close,at

three different places, the annular space comprised between the outerwalls of the pipes and the wall of the borehole, so as to bound, in aportion of the borehole, three separate adjacent cavities, the abovesaidpipes being provided with openings situated so that one of the pipescommunicates with the intermediate cavity whilst the other pipecommunicates with the two end cavities, a flow meter being provided atleast in the pipe communicating with the intermediate cavity.

In another embodiment, the apparatus is characterised by the fact thatit comprises a single tubular pipe adapted to be introduced into theborehole and at least three closures adapted to close, at threedifferent places, the annular space comprised between the outer wall ofthe pipe and the wall of the borehole, so as to bound, in a portion ofthe borehole, three separate adjacent cavities, the abovesaid pipe beingprovided with openings terminating in each of the three cavities, twoflow meters being provided, in the said pipe, respectively at the twoends of the intermediate cavity, for the measurement of the flow-rate ofliquid at the inlet and at the outlet of this cavity.

The invention consists, apart from the features mentioned above, ofcertain other features which are preferably used at the same time andwhich will be more explicitly considered below with reference topreferred embodiments of the invention which will now be described inmore detailed manner with reference to the accompanying drawing, butwhich are not of course to be regarded as in any way limiting.

FIG. 1 of this drawing is a diagram illustrating a determination of thepermeability characteristics of a soil carried out by the methodaccording to the invention.

FIG. 2 is a diagrammatic partial longitudinal section, of a first typeof apparatus for the application of the method according to theinvention.

FIG. 3 is a cross-section of the pipes of the apparatus of FIG. 2.

FIG. 4 shows similarly to FIG. 2, another type of apparatus.

FIG. 5, lastly, is an enlarged diagrammatic section of v a piezometer.

Referring to FIG. 1 it is seen that for determining the permeability ofamedium, constituted by soil S or rock, a flow of liquid caused by theinjection of this liquid into a borehole T is used. In certain'cases,especially when the determination of the permeability of the soil takesplace in a zone of the latter situated below the phreatic layer or watertable, this determination can be effected by the pumping of water fromthe soil into the borehole, instead of the aforesaid injection.

The total flow-rate of the liquid injected into the borehole T isdenoted by the letter O which, in FIG. 1, is arranged at the side of anarrow indicating the direc' tion of flow of the liquid into the boreholeT.

A portion P of the borehole T is divided, along the axis of this hole,into three adjacent cavities respectively l, 2 and 3.

In FIG. 1, the end cavity 3 most distant from the inlet of the boreholeT is bounded, on one side, by the bottom of this borehole. However, thepart P does not necessarily extend to the bottom of the borehole but canbe bounded by a closure (not shown), especially if the flow-rate in thiscavity is very great.

The three cavities are separated from one another, and the two endcavities 1 and 3 constitute protective cavities which enclose theintermediate cavity 2 constituting the measuring cavity.

The injection of liquid into the borehole T enables a flow to be causedin each of the cavities 1, 2 and 3 and in the corresponding regions ofthe soil. The flow E of the intermediate cavity 2 can be characterisedindependently of the flows E, and E of the end cavities.

The flow-rate of liquid O in the intermediate cavity 2 is measured.There is also measured the pressure of the liquid in this cavity and inthe region of the soil S corresponding to thiscavity, the measurementsin the soil S being carried out at known distances r from the axis ofthe borehole. These pressure measurements are effected by means ofpiezometers 4 introduced into the soil and connected to the surface ofthe latter.

FIG. 5 shows a preferred embodiment of a piezometer 4.

The latter comprises a tube 4a, extended at its lower end by a strainer4b. Annular closures 4c and 4d are provided around the tube, at thelongitudinal ends of the strainer 4b. The closure 4d closes the lowerlongitudinal end of this strainer 4b.

The piezometer 4 is introduced into an auxiliary borehole parallel tothe principal borehole but of smaller diameter. The zone of theauxiliary borehole comprised between the closures 4c, 4d collects liquidcoming from the intermediate cavity 2. This liquid rises in the tube 4aunder the effect of the pressure. In measuring the height of the rise ofthe liquid in the tube 4a, by means of anelectric probe for example, thepressure of the liquid in the zone of the auxiliary borehole concernedis determined. This zone and the distance between the closures 4c and 4dcan be much reduced so that the measurement of pressure is carried outsubstantially at one point in the soil.

From the results of the measurements, it is possible, by means ofmathematical formulae, to deduce the permeability of the soil in thedirection of flow E In the case of an isotropic medium and of a flow Ein directions perpendicular to the axis of the borehole, the differencein hydraulic potential A Q at two points of the soil S distant by r andr,, from the axis of the borehole is connected with the flow-rate liquidQ by the following formula:

(I) in which formula:

Q, is the flow-rate measured,

L is the dimension of the cavity 2 along the axis of the borehole,

Kr is the average permeability of the soil, in a plane perpendicular tothe axis of the borehole.

1 is equal to the outer potential existing at the center ofthe borebefore the test; to a first approximation this term d can be neglected.

FIG. 2 shows a first type of apparatus enabling the application of themethod discussed above.

The measuring apparatus or probe 5 comprises two distinct tubular pipes6 and 7 arranged side by side and tangential along a rectilineargenerator. These pipes are arranged in two (or more) superposed sleeves8 and 9. However, the pipes 6 and 7 could be inserted into the boreholeT, without being surrounded by sleeves 8 and 9.

The probe 5 comprises also at least three closures 10 and 12 adapted toclose, at three different places sepacomprised between the wall of theborehole T and the 1 outer walls of the sleeves 8 and 9. In this way,there are obtained three adjacent cavities l, 2 and 3.

Preferably, the closures are of the pneumatic type and constituted byinflatable .air chambers. Compressed air pipes (not shown) are providedin the borehole T for the inflation of these closures.

3 The upper closure 10, that is to say that situated at the sideof'theinlet of the borehole T, has a length along the axis of the borehole,greater than that of the other closuresl In fact, this closure issubjected to pressures very different at its two ends since on one sideit is subjected to the liquid pressure occurring in the cavity l whilst,on the other side, it is subjected simply to the atmospheric pressureincreased by that of the columns of water possibly present in the boreabove the closure. There is given, for example, to the length of j theclosure 10, a length three times that of the closures 11 and 12.

Each sleeve 8 and 9 is formed by a cylindrical envelope generallymetallic or of plastics material. At its two ends, this envelope isconnected in fluidtight manner by a circular ring, to the outer wall ofthe pipes 6 and 7. The contact zone of the sleeve 8 with the sleeve 9 iscovered by the closure 11.

The end zone of the sleeve 8 turned towards the inlet of the borehole Tis surrounded by the closure 10. The wall of the sleeve 8 comprisesorifices 13 enabling a radial flow of the liquid towards the soilS. Thewall of the conduit 6 comprises, in the zone comprised axially betweenthe closures 10 and 11, further orifices 14, enabling a radial flow ofliquid towards the soil S. The pipe 6, in addition, opens through anorifice 14a into the end cavity 3. The supply of liquid to the cavitiesI and 3 is hence ensured by the single pipe 6. In a modification, therecould be provided a supply pipe belonging to each cavity 1 and 3.

The wall of the pipe 7 comprises orifices 15, in the zone situatedaxially between the closures 11 and 12, which enable a radial flow ofthe liquid towards the soil S. The wall of the sleeve 9 comprisesorifices 16 enabling the passage of this liquid towards the soil S. Thesleeve 9 and the pipe 7 are closed at their end turned towards thebottom of the hole T so that mixing of the fluids injected,respectively, through the pipes 6 and 7 cannot occur there.

A flow meter 17 is provided in the pipe 7, at the surface, this flowmeter enabling the flow-rate Q of the flow E to be known. Possibly,there could be provided another flow meter in the pipe 6 which wouldindicate the total of the flow rates of the flows E and E An approximatevalue of the pressure of the liquid in the cavity 2 can be obtained bythe measurement, at the surface, of the pressure of theliquid in thepipe 7. However, by reason of the load losses which can be high if thelength ofthe pipes 6 and 7 is great and if the flow-rates are high, itis preferable to measure the pressures directly in the test cavities l,2 and 3, by providing either pressure detectors (not shown) lodged inthese cavities, or auxiliary pipes 18, 19 (FIG. 3) of smallcross-section, connecting respectively the cavity 2 and the cavities 1and 3 to the surface of the soil. The pipes 18 and 19, in which no flowtakes place. can be of small section without introducing load losses.

It will be noted, to conclude with this first type of apparatus, thatthe two pipes 6 and 7 can beproduced in a different form from thatdescribed with reference to FIGS. 2 and 3. For example, these two pipescan be obtained by a coaxial double casing, or by a single tube divided,in the direction of the length, by a partition extending in a diametricplane of this tube, the said partition separating the tube into twoindependent parts of which the cross-sections are semi-circles.

Referring to FIG. 4, there can be seen a second type of probe comprisinga single tubular pipe 20. This probe comprises, as in the case of FIGS.2 and 3, the three closures 10, 11 and 12 bounding the cavities l, 2 and3.

The pipe 20 comprises openings 21 in the portion of its wall comprisedbetween the closures and 11 and openings 22 in the portion of its wallcomprised between the closures I1 and 12. The pipe opens at its lowerend through an opening 23 into the cavity 3.

The first flow meter 24 is provided at the inside of the pipe 20; it issituated in the axial direction of the borehole, at the level of theclosure 11, that is to say at the separation of the cavities 1 and 2.This flow meter 24 is hence adapted to measure the flow-rate of liquidentering the cavity 2.

A second flow meter 25 is provided, at the level of the closure 12,between the cavities 2 and 3. This flow meter 25 is adapted to measurethe flow-rate of the liquid which enters the cavity 5. The flow-rateofliquid Q of the flow E is hence equal to the difference of theflow-rates measured respectively by the flow meter 24 and the flow meter25. These flow meters are of the electrical transmission type and areconnected to the surface by electrical cables 26 adapted to transmit theinformation provided by these flow meters.

A piezometric cell 27, also of the electrical transmission type, isprovided in the cavity 2 for the measurement of the liquid pressure inthis cavity. The pressure could be also measured by an auxiliarypiezometric tube.

It will be noted that the probe of FIGS. 2 and 3 enables theestablishment in the cavity 2 of a pressure different from that whichexists in the cavities l and 3, so that, as will be seen in thefollowing, there can be introduced, in the course of a test, thepermeability of the soil in a direction parallel to the axis of theborehole. On the other hand. the probe of FIG. 4, due to the fact thatit only comprises a single pipe 20 for the simultaneous supply ofthecavities I, 2 and 3, only enables operation at the same pressure in thesaid cavities.

The two probes can be moved in the borehole for measurements indifferent places.

To carry out correct measurements of the permeability of a mediumconsidered as continuous, the axis of the borehole is oriented in thedirection of principal permeability, which must hence be assumed, sothat all the principal permeabilities do not come into playsimutaneously in the course of a test.

The flow E coming from the measurement cavity 2, being a flat radialflow at right angles to a principal direction of permeability, only thetwo other principal permeabilities will effect the flow-rate of thisflow.

The abovesaid direction of principal permeability is an assumeddirection deduced from geological knowledge. For example, forsedimentary terrains it is known that a direction ofprincipalpermeability is perpendicular to the sedimentary layers whilst the twoother directions of principal permeability are parallel to these layers.

In the case of a discontinuous medium, for example in the case offissured rocks. with three systems of parallel fissures, the directionof a bore, to test one of the fissured systems, will be taken parallelto the intersection of the planes of the two other systems of fissures.

Then, by keeping the pressures in the cavities 1, 2 and 3 equal, thereis effected a flat radial flow E in a certain region, of which the flowlines are at right angles to the assumed principal directions. On theother hand, the flows E E corresponding to the protective cavities 1 and3, are not entirely of the flat radial type.

In the case of a simple test, there is measured during this test, thehydraulic load q in the cavity 2. This hydraulic load is constant inthis cavity and especially for any point taken on the lateral wall ofthis cavity. 1 represents therefore the hydraulic load in the soil at adistance from the axis of the borehole equal to the radius r, of thisborehole.

There is carried at least one other pressure measurement to be able touse the formula (I) (or a formula more appropriate to theexperimentalconditions).

This pressure measurement can be replaced by a measurement, before thetest, in the borehole. The result of this measurement 1 (static level ofthe phreatic layer) corresponds, during the test, to the pressure whichexists at a point of the soil situated at a distance from the axis ofthe borehole, equal to the radius of the action of the test. This actionradius can be calculated empirically.

If the medium is continuous but is not isotropic, the value K obtainedby the formula (I) is equal to the geometric mean ofthe principalpermeabilities in the plane perpendicular to the axis of the bore.

It is possible, by effecting several pressure measurements in the soilalong vector radii, starting from the axis of the borehole, and atdifferent polar angles, to trace ellipses corresponding to equipotentiallines. By determining the directions of the axes of these ellipses,there is determined the principal directions of permeability in theplane at right angles to the axis of the borehole.

These directions of principal permeability being determined, ananalytical calculation, using the result of the measurement of theflow-rate of the flow E enables the determination of the values of thetwo principal permeabilities in a plane perpendicular to the axis of theborehole.

Whilst keeping equal the pressures in the cavities 1, 2 and 3, it ispossible to deduce the third principal permeability, that is to say thepermeability along the axis of the borehole, while measuring the totalflow-rate in the cavities l, 2 and 3, which brings into play thepermeability along the axis of the borehole, and by bringing into playthe results of the first test phase.

There could, however, in the case where the probe of FIG. 2 is used, bebrought into play in more sensitive manner the permeability along theaxis of the borehole by establishing a difference of pressure between,on one hand, the cavities l and 3 and, on the other hand, the cavity 2.

To improve the accuracy of the measurements, that is to sayparticularly, in order that the flow E may depart as little as possiblefrom a theoretical radial plane flow, the length L, along the axis ofthe borehole, of the measuring cavity 2, is limited.

To effect this limitation flow lines have been drawn, in a plane passingthrough the axis of the borehole, from probable hypotheses.

According to the distance to the axis of the borehole,

at which the pressure measurements are made, the length of the measuringcavity is limited so that the flow lines coming from this cavity, do notseparate, angularly, beyond a predetermined limit, 10 for example) fromdirections at right angles to the axis of the borehole.

For a study in depth of the permeability characteristics of a continuousmedium, bores are made along the three presumed principal directions ofpermeability and measurements are made along these three directions.

In the case of a discontinuous medium constituted by fissured rockshaving three parallel families of fissures, the bores are effected alongthe directions of the intersections of two families of parallelfissures.

The method and the apparatuses according to the invention can be used innumerous fields where problems of flow of fluids in porous or fissuredmedia arise, as for example in civil engineering (subterranianhydraulics of soils and of rocks), in geohydrology, in the field ofmining operations, of petroleum production, of techniques relating toartificial porous media such as filters for the chemical industry orceramics, etc

l'claim:

1. Method for determining the permeability characteristics of a porousor fissured medium, comprising forming a borehole in situ in the medium,dividing said borehole along its axis into three adjacent cavities,separated from one another and comprising two protecting end cavitiesenclosing an intermediate measuring cavity, producing a flow of a liquidin each of the cavities and in the corresponding regions ofcorresponding medium, the direction of flow with respect to the mediumbeing the same for each cavity, and effecting measurement oftheflow-rate of liquid flowing in the intermediate cavity and measurementsof the liquid pressure in said intermediate cavity and in thecorresponding region of the medium, at known distances from the axis ofthe borehole and determining the permeability characteristics from saidflow-rate and said liquid pressure measurements..

2. Method according to claim 1, to determine the permeabilitycharacteristics of a fissured discontinuous medium having three familiesof parallel fissures, comprising orienting the-axis of the boreholeparallel to the direction of the intersection of two of said families offissures. 1

3. Measuring apparatus for determining'the permeability characteristicsof a porous or fissured medium. comprising at least two separate mainpipes adapted to be introduced into a borehole formed in said medium andat least three closures adapted to close, at three different places, theannular space comprised between the outer walls of the pipes and thewall of the borehole, in such a way as to define, in a part of theborehole, three adjacent separate cavities, the abovesaid pipes beingprovided with openings so-situated that one of the pipes communicateswith the intermediate cavity and is closed at its lower end turnedtowards the bottom of the borehole so that mixing with fluid entering orleaving the other pipe does not occur while the other pipe communicateswith the two end cavities, a flow meter being provided at least in thepipe communicating with the intermediate cavity.

4. Measuring apparatus according to claim 3, comprising auxiliary pipesconnected to the surface of the soil and terminating respectively in theend cavities and in the intermediate cavity and enabling the pressure tobe measured in these cavities.

5. Measuring apparatus according to claim 4, wherein the two main pipesand the two auxiliary pipes are arranged side by side.

6. Measuring apparatus according to claim 3, wherein the closures are ofthe pneumatic type.

7. Measuring apparatus according to claim 6, wherein the closuresituated at the extremity of an end cavity, distant from theintermediate cavity, has a greater length than that of the closuressituated at the two extremities of the intermediate cavity.

1. Method for determining the permeability characteristics of a porous or fissured medium, comprising forming a borehole in situ in the medium, dividing said borehole along its axis into three adjacent cavities, separated from one another and comprising two protecting end cavities enclosing an intermediate measuring cavity, producing a flow of a liquid in each of the cavities and in the corresponding regions of corresponding medium, the direction of flow with respect to the medium being the same for each cavity, and effecting measurement of the flow-rate of lIquid flowing in the intermediate cavity and measurements of the liquid pressure in said intermediate cavity and in the corresponding region of the medium, at known distances from the axis of the borehole and determining the permeability characteristics from said flow-rate and said liquid pressure measurements.
 2. Method according to claim 1, to determine the permeability characteristics of a fissured discontinuous medium having three families of parallel fissures, comprising orienting the axis of the borehole parallel to the direction of the intersection of two of said families of fissures.
 3. Measuring apparatus for determining the permeability characteristics of a porous or fissured medium, comprising at least two separate main pipes adapted to be introduced into a borehole formed in said medium and at least three closures adapted to close, at three different places, the annular space comprised between the outer walls of the pipes and the wall of the borehole, in such a way as to define, in a part of the borehole, three adjacent separate cavities, the abovesaid pipes being provided with openings so-situated that one of the pipes communicates with the intermediate cavity and is closed at its lower end turned towards the bottom of the borehole so that mixing with fluid entering or leaving the other pipe does not occur while the other pipe communicates with the two end cavities, a flow meter being provided at least in the pipe communicating with the intermediate cavity.
 4. Measuring apparatus according to claim 3, comprising auxiliary pipes connected to the surface of the soil and terminating respectively in the end cavities and in the intermediate cavity and enabling the pressure to be measured in these cavities.
 5. Measuring apparatus according to claim 4, wherein the two main pipes and the two auxiliary pipes are arranged side by side.
 6. Measuring apparatus according to claim 3, wherein the closures are of the pneumatic type.
 7. Measuring apparatus according to claim 6, wherein the closure situated at the extremity of an end cavity, distant from the intermediate cavity, has a greater length than that of the closures situated at the two extremities of the intermediate cavity. 